Polymers that exhibit a high dielectric constant and generate a large electric displacement or/and mechanical actuation due to an external electric field have attracted a great deal of attention and have been used as capacitors, transducers, actuators and sensors. Currently, most of the commercial applications for high dielectric and ferroelectric materials are based on piezoceramics and magnetostrictive materials, despite the fact that they exhibit many deficiencies, such as low strain levels, brittleness, heavy weight, and high processing temperatures. Moreover, these materials exhibit processing difficulties when producing parts having complicated shapes. In contrast, ferroelectric polymers exhibit many desirable properties, such as flexibility, light weight, high mechanical strength, an ability to be processed readily into large area films, and an ability to be molded readily into a variety of configurations. Despite the advantages over ceramic materials, most polymers suffer the disadvantage of having a low electric field sensitivity, in terms of dielectric constant, electric displacement, piezoelectric coefficient, electromechanical coupling coefficient and field induced strain, which limit their applications.
In the past decade, most of the research activities involving high dielectric and ferroelectric polymers have focused on fluoropolymers, especially semicrystalline VDF/TrFE copolymers. Many research efforts have been devoted to a general goal of reducing the energy barrier for ferroelectric-paraelectric (Curie) phase transition, and for generating a large and fast electric-induced mechanical (piezoelectric) response at ambient temperature. Although VDF/TrFE copolymers (stretched film poled at 120° C.) exhibit a relatively high piezoelectric constant (Koga, et al. J. Appl. Phys., 59, 2142, 1986), the response of the dipoles to an electric field is very slow at ambient temperature, and the polarization hysteresis loop (polarity vs. electric field) of the copolymer is very large. A VDF/TrFE copolymer comprising 55 mole % VDF and 45 mole % TrFE, which exhibits the narrowest polarization hysteresis loop and lowest Curie temperature of the copolymers in the VDF/TrEF family (Higashihata, et al. Ferroelectrics, 32, 85, 1981), still exhibits a significantly wide hysteresis loop.
The connection between crystalline structure and electric properties led many to attempt to alter the copolymer morphology by creating non-equilibrium states; and a number of such attempts resulted in ferroelectric polymers that exhibit somewhat improved electric responses. Such attempts have included, for example, subjecting ferroelectric polymers to mechanical deformation (Tashiro, et al. Macromolecules, 21, 2463, 1988, and 23, 2802, 1990), electron-radiation (Daudin, et al. J. Appl. Phys., 62, 994, 1987; Zhang et al. U.S. Pat. No. 6,423,412), uniaxial drawing (Furukawa, et al. Japanese Journal of Applied Physics, 29, 675, 1990), crystallization under high pressure (Yuki, et al. Jpn. J. Appl. Phys., 37, 5372, 1998), and crystallization under high electric field (Ikeda, et al. Jpn. J. Appl. Phys., 31, 1112, 1992).
Chung et al. (U.S. Pat. No. 6,355,749, Ferroelectrics Letters 28, 135, 2001) showed an alternative method to change the crystalline domains and create relaxor ferroelectric behavior of VDF/TrFE copolymers by introducing a small amount (<10 mole %) of bulky ter-monomer units, such as chlorotrifluoroethylene (CTFE) units, into the copolymer with uniform molecular structure. The resulting terpolymers are completely solution and melt processible and form a desirable film morphology with uniform nano-crystalline domains that have Curie (polar-nonpolar crystalline phase) transition at about ambient temperature. The polarization hysteresis loop of the terpolymer also became very narrow. Therefore, these terpolymers exhibit very high dielectric constant (∈>70) at ambient temperature and fast and high electromechanical response (>5%) induced by external electric field.
Progress in terpolymers caused some researchers to investigate terpolymer composite materials (Nature 419, 284, 2002; J. Appl. Polym. Sci. 82, 70, 2001) with the objective to further enhance their dielectric properties, especially in two areas, (a) increasing the dielectric constant, so that a lower electric voltage is needed for large electromechanical response and (b) reducing dielectric loss during phase transition, which is a concern in device designs, especially for long term usages. The preparation of these composites was undertaken by simple blending of the terpolymer with some high dielectric materials, including copper phthalocyanine (CuCy) organic molecule (J. Appl. Polym. Sci. 82, 70, 2001, Nature 419, 284, 2002, Macromolecules 38, 2247, 2005). However, the low surface energy and non-stick properties of fluoropolymers result in the incompatible blends between fluoropolymers and high dielectric materials. The incompatibility is a major obstacle in preparing a uniform composite film (even>40 μm thickness) with good mechanical properties, which are essential for many electric applications. Overall, it is still elusive to prepare high performance dielectric materials.
Generally, methods of preparing fluoropolymers have been by free radical emulsion and suspension polymerization processes in aqueous solution using a batch reactor (F. J. Honn, et al. U.S. Pat. No. 3,053,818; J. E. Dohany, et al. U.S. Pat. No. 3,790,540; T. Sakagami, et al. U.S. Pat. No. 4,554,335; J. Sako, et al. U.S. Pat. No. 4,577,005; H. Inukai, et al. U.S. Pat. No. 5,087,679; H. Freimuth, et al. Polymer, 37, 831, 1996). The combination of heterogeneous reaction conditions, limited gas diffusion of the monomers in water, significant difference in comonomer reactivity ratios, and high monomer conversion in batch reactions inevitably results in co- and ter-polymers having a broad compositional distribution and inhomogeneous crystalline domains. In addition, it is also difficult to completely remove emulsifying and suspending agents (particularly those containing polar groups), after emulsion and suspension polymerization processes, respectively, which are detrimental to the resultant dielectric properties of the final product.
Chung (Macromolecules 35, 7678, 2002; 39, 5187, 2006; and 39, 4268, 2006) has also disclosed a method involving a solution or bulk polymerization process at ambient temperature using control radical initiators, such as organoborane/oxygen adducts, which form the co- and ter-polymers with narrow compositional and molecular weight distributions. The low temperature and relatively slow polymerization process also minimize the safety concerns, usually associated with bulk polymerization of fluoromonomers. This homogeneous polymerization has prepared co- and ter-polymers with narrow molecular weight and composition distributions and high purities, without the need for an emulsifying or suspending agents. Several control organoborane/oxygen radical initiators were discovered, which exhibited living radical polymerization characteristics, with a linear relationship between polymer molecular weight and monomer conversion to producing block copolymers by sequential monomer addition (Chung, et al. J. Am. Chem. Soc., 118, 705, 1996).
While methods for preparing certain fluoropolymers having a high dielectric constant are known (see, e.g., U.S. Pat. No. 6,355,749), it is believed that certain fluoropolymers having high dielectric constants coupled with reactive functional groups that can provide good chemical reactivity have not been recognized. The interactive property provides the fluoropolymers with many advantageous features, including the incorporation of high dielectric organic/inorganic materials with uniform morphology to further enhance their dielectric and ferroelectric properties, the ability of forming a 3-D network that exhibits high mechanical strength, high breakdown voltage under extremely high external electric field conditions, and good adhesion to electrodes.
The chemistry for preparing chain end functionalized fluoropolymer is limited. A few examples of controlling chain end structure include the use of a functional initiator, which was reported by Rice and Sandberg at the 3M Company (see U.S. Pat. No. 3,461,155). They reported the preparation of low molecular weight VDF/HFP elastomers containing two ester terminal groups by using a diester peroxide initiator. The average functionality of the resulting telechelic VDF/HFP elastomer was not reported. However, it is logical to expect some difficulties in preparing a telechelic structure by using a free radical polymerization due to the fact that such techniques typically involve many side reactions in the termination step. Recently, Saint-Loup et al. (see Macromolecules, 35, 1542, 2002) also attempted to prepare chain end functionalized VDF/HFP elastomers containing two opposing hydroxy terminal groups by using hydrogen peroxide as an initiator. Several advantages of using hydrogen peroxide initiator include cost, high reactivity, and directly forming hydroxy terminal groups. However, many side reactions also occur in this polymerization, and the final product contains not only hydroxy terminal groups but also carboxylic acid terminal groups, as well as some unsaturated terminal groups.
A widely used method for preparing chain and functionalized fluoropolymers was developed by Daikin Corp. in the late 1970's and early 1980's (U.S. Pat. Nos. 4,158,678 and 4,361,678), which includes an iodine transfer polymerization (ITP) step to prepare fluoropolymers having two terminal iodine groups. The chemistry is based on the combination of a reversible addition-fragmentation chain transfer (RAFT) process and an α,ω-diiodoperfluoroalkane (I—RF—I) chain transfer agent, where RF represents CF2CF2, CF2CF2CF2CF2, etc. The active CF2—I groups are located at both ends of the polymer chain and maintain similar reactivity despite the growing polymer chain. The polymerization characteristics are usually demonstrated by an increase of molecular weight with conversion of monomer and relatively narrow molecular weight distribution (Mw/Mn<2). This reaction process has led to a commercial product, i.e. diiodo-terminated VDF/HFP elastomers with the trade name Dai-E1®, which is useful as a sealing material for O-ring, gaskets, tubes, valves and bellows, as well as useful in linings, protective gloves, and shoes.